In field applications, chemical analysis instruments may be confronted with various complex mixtures regardless of indoor or outdoor environments. Such mixtures may cause instrument contamination and confusion due to the presence of molecular interferents producing signatures that are either identical to that of the chemical compounds of interest or unresolved by the analytical instrument due to its limited resolution. An interferent can also manifest its presence by affecting the limit of detection of the compound of interest. A multi-stage analysis approach may therefore be used to reduce the chemical noise and produce enough separation for deterministic detection and identification. The multi-stage analysis may include either a single separation technique such as mass spectrometry (MS) in MSn instruments or a combination of different separation techniques. These are called orthogonal techniques since, even though they may operate in tandem, they measure different properties of the same molecule by producing multi-dimensional spectra hence increasing the probability of detection and accuracy of detection. For field instruments, such techniques may be physically and operationally integrated in order to produce complementary information hence improving overall selectivity without sacrificing speed and sensitivity.
In the area of trace explosives detection, ion mobility spectrometry may be commonly used at passenger checkpoints in airports. The technique relies on the availability of sufficient explosives residue (particles and/or vapor) on the passenger skin, clothing, and personnel items to signal a threat. The assumption being that due to their high sticking coefficient it is difficult to avoid contamination by explosives particles during the process of handling a bomb. The same high sticking coefficient results in extremely low vapor pressures and hence makes their detection difficult. The acquisition of vapor and/or particle samples may be achieved by either swiping “suspect” surfaces of luggage or persons, or in the case of portals and/or by sending pulses of compressed air intended to liberate particles off the person's clothing, skin, shoes etc. . . . . In both cases the sample is introduced into an ion mobility spectrometer (IMS) for analysis.
Ion mobility spectrometry utilizes relatively low electric fields to propel ions through a drift gas chamber and separate these ions according to their drift velocity. In IMS, the ion drift velocity is proportional to the field strength at low electric fields (˜200 V/cm), and thus an ion's mobility (K) is independent of the applied field. In the IMS both analyte and background molecules are typically ionized using radioactive alpha or beta emitters and the ions are injected into a drift tube with a constant low electric field (300 V/cm or less) where they are separated on the basis of their drift velocity and hence their mobility. The mobility is governed by the ion collisions with the drift gas molecules flowing in the opposite direction. The ion-molecule collision cross section depends on the size, the shape, the charge, and the mass of the ion relative to the mass of the drift gas molecule. The resulting chromatogram is compared to a library of known patterns to identify the substance collected. Since the collision cross section depends on more than one ion characteristic, peak identification is not unique. IMS systems measure a secondary and less specific property of the target molecule—the time it takes for the ionized molecule to drift through a tube filled with a viscous gas under an electric field—and the identity of the molecule is inferred from the intensity vs time spectrum. Since different molecules may have similar drift times, IMS inherently has limited chemical specificity and therefore is vulnerable to interfering molecules.
High-field asymmetric waveform ion mobility spectrometry (FAIMS) is an emerging detection technology which can operate at atmospheric pressure to separate gas-phase and detect ions, as first described in detail by Buryakov, I. A.; Krylov, E. V.; Nazarov, E. G.; Rasulev, U. K., International Journal of Mass Spectrometry and Ion Processes 1993, 128 (3), pp. 143-148, which is incorporated herein by reference. FAIMS separates ions by utilizing the mobility differences of ions at high and low fields. Compared to conventional ion mobility, FAIMS operates at much higher fields (˜10,000 V/cm) where ion mobilities become dependent on the applied field and are better represented by Kh, a non-constant high-field mobility term. Variations in Kh from the low-field K, and the compound-dependence of that variation aids FAIMS in its separation power. FAIMS utilizes a combination of alternating current (AC) and direct current (DC) voltages to transmit ions of interest and filter out other ions, thus improving specificity, and decreasing the chemical noise. FAIMS can reduce false positives, since two different compounds having the same low-field mobility can often be distinguished in a high-field environment.
Ions are separated in FAIMS by their difference in mobility at high (Kh) and at low (K) electric fields. At a constant gas number density, N, the non-linear dependence of an ion's mobility in high electric fields can be described byKh(E)=K0[1+α(E/N)2+β(E/N)4+ . . . ]  Equation (1)    where K0 is the ion mobility coefficient at zero electric field and α and β describe the dependence of the ion's mobility at a high electric field in a particular drift gas.3 This equation is an infinite series, but at realistic field intensities the terms above the 4th order become insignificant. FAIMS cells are commonly comprised of two parallel electrodes, one typically held at a ground potential while the other has an asymmetric waveform applied to it. A commonly used asymmetric waveform, described by V(t) in Equation 2, includes a high-voltage component (also referred to as V1 or dispersion voltage [DV]) which lasts for a short period of time (t1) relative to a longer lasting (t2) low-voltage component (V2) of opposite polarity. Most FAIMS work up to date has employed a sinusoidal wave, plus its first harmonic at twice the frequency, as shown in Equation 2, where co is the frequency in radians per second.V(t)=(0.61)V1 sin(ωt)+(0.39)V1 sin(2ωt−π/2) . . .  Equation (2)    The waveform is constructed so that the voltage-time product applied to the electrode is equal to zero, as displayed in Equation 3.V1t1+V2t2=0  Equation (3)
At high electric fields, the application of this waveform will cause an ion to experience a net drift toward one of the electrodes. Ions passing between the electrodes encounter this displacement because the ion's mobility during the high-voltage component (Kh) is different than that from the low-voltage mobility (K). In other words, the ion will move a different distance during the high-voltage portion than during the low-voltage portion. This ion will continue to migrate towards one of the electrodes and subsequently be lost unless a DC compensation voltage (CV) is applied to offset the drift. The CV values required to offset the drift of different ions will be different if the Kh/K ratio of the ions are different. Thus, a mixture of compounds can be successfully separated by scanning the CV, allowing each compound to transmit at its characteristic CV, creating a CV spectrum.
When higher electric fields are applied to the FAIMS electrodes, an ion can have three possible changes in ion mobility. The mobility of type A ions increases with increasing electric field strength, the mobility of type C ions decreases, and the mobility of type B ions increases initially before decreasing at yet higher fields. Most low-mass ions (<m/z 300) are type A ions, whereas most high-mass ions are type C ions.
In addition to stand-alone use, FAIMS devices may be used to filter ions prior to analyses with mass spectrometry (MS) devices and/or drift tube IMS devices, and reference is made, for example, to U.S. Patent App. Publication No. 2010/0207022 A1 to Tang et al, published Aug. 19, 2010, entitled “Platform for Field Asymmetric Waveform Ion Mobility Spectrometry with Ion Propulsion Modes Employing Gas Flow and Electric Field,” which is incorporated herein by reference. Tang et al. principally discuss multiple device instruments stages using a FAIMS device coupled to a subsequent device, such as an IMS or MS device, and in which the FAIMS device may be rapidly switched on or off to enable more sensitive analyses using the other stage(s). Paragraph [0010] of Tang et al. suggests that in such multiple device instrument stages it is possible for the other stage(s) to precede the FAIMS device; however, this discussion in Tang et al. is still directed towards the goal of providing a method for effective, rapid and convenient switch-off of the FAIMS separation in hybrid platforms to enable more sensitive analyses using the other stage(s).
Accordingly, it would be desirable to provide a system that provides for flexible operation to handle a variety of detection scenarios and that provides for enhanced chemical detection and identification capabilities.